The present invention relates to a lift cylinder and a mast assembly of forklifts.
A typical forklift has lift cylinders for lifting and lowering a fork, which is supported by a mast assembly. There are several types of mast assemblies. In one type of mast assembly, a fork is lifted and lowered together with inner masts. There is also a full free type mast assembly, which includes a full free cylinder and mast cylinders. The full free cylinder lifts and lowers the fork without moving inner masts, while the mast cylinders lift and lower the inner masts.
The mast cylinders extend and retract the mast assembly, and the full free cylinder lifts and lowers a lift bracket, to which the fork is secured, relative to the inner mast. When lifting the fork, the mast cylinders start extending the mast assembly after the full free cylinder reaches its maximum length. When lowering the fork, the full free cylinder starts retracting after the mast cylinders retract to the minimum length.
In a typical forklift, an operator manipulates a height control lever to lift a fork to a certain height. In addition to the height control lever, some forklifts (for example, one described in Japanese Unexamined Patent Publication No. 7-2496) are equipped with an automatic lifting apparatus for lifting the fork. The operator touches a control panel to actuating the automatic lifting apparatus, which, in turn, automatically lifts the fork to a desired height. The apparatus includes a controller that controls the lift cylinders. The controller detects the height of the fork based on a signal from a height sensor and stops the lift cylinders when the fork reaches a target height. When a forklift is used indoors, or in a place with a ceiling, careless lifting of the fork may damage the ceiling. To avoid this problem, some forklifts are equipped with a height limiting apparatus. The height limiting apparatus detects the height of the fork and prevents the fork from being lifted beyond a predetermined height.
A typical forklift has a reel type height sensor for continuously detecting the height of the fork. A reel type height sensor includes a wire, one end of which is connected to the fork, a reel, about which the wire is wound, and a potentiometer for detecting a rotation amount of the reel.
The reel increases the size of the height sensor. Also, the wire, which is exposed, can be damaged by contact with foreign objects. Further, dust may cause the potentiometer to malfunction. Accordingly, frequent maintenance of the sensor is required. Especially, when the forklift is used at a place near the sea or in a condition where brine is used, the potentiometer can easily malfunction.
In some forklifts, a rotary encoder is used for detecting the height of a fork instead of a reel type height sensor. In this case, a rack is formed on a mast and a pinion is mounted on the fork to integrally move with the fork. The rotary encoder detects the rotational amount of the pinion, which rotates as the fork is lifted or lowered. The rotary encoder can be an absolute type or an incremented type. An absolute type encoder is costly. On the other hand, an incremented type encoder, which is less costly than an absolute type encoder, requires that a referential position of the fork be accurately detected. If the referential position is set to the highest position of the fork, the fork must be lifted to the highest position every time the forklift starts operating, which wastes energy. Therefore, the referential position is preferably the lowest position of the fork. Accordingly, a sensor for accurately detecting the lowest fork position is needed.
In a typical forklift, an axle for supporting the rear wheels pivots relative to a body frame for stabilizing the body frame. However, under certain conditions, tilting of the axle will destabilize the forklift. To avoid this problem, Japanese Unexamined Patent Publication No. 58-167215 describes a forklift that locks the rear axle under certain circumstances. The mechanism includes a weight detector and a height detector. The weight detector detects whether the load on the fork is greater than a predetermined threshold value and the height detector detects whether the fork height is greater than a predetermined threshold value. The mechanism locks the rear axle against pivoting when the weight of the load on the fork and the height of the forks exceed the threshold values.
In a typical forklift, the mast assembly is tilted when loading or unloading the fork. The maximum speed of tilting is limited based on the fork height and the load weight on the fork.
In a forklift having a full free type mast assembly, setting the referential height of the fork to a height corresponding to the maximum length of the full free cylinder allows the pivoting of the rear axle and the tilting speed of the mast assembly to be easily and effectively controlled. A sensor for detecting the maximum length of the full free cylinder can be mounted on the full free cylinder. However, since the full free cylinder is lifted and lowered integrally with inner masts, the sensor on the full free cylinder must be connected to a controller by long wires. Further, when the inner masts are at the lowest position, the loose wires must be prevented from interfering with other parts. The long wires are a particular concern in a full free type triplex mast assembly, which has inner masts, middle masts and outer masts.
The time at which the full free cylinder reaches its maximum length can be indirectly detected without monitoring changes in the length of the full free cylinder. That is, the maximum length of the full free cylinder can be detected by measuring time from when the mast cylinders start extending.